This invention relates, in general, to electrostatographic imaging systems, and, in particular to improved developer materials and their use.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
Many methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552 is known as "cascade" development. In this method a developer material comprising relatively large carrier particles having finely divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the electrostatic latent image bearing surface. The composition of the toner particles is so chosen as to have a triboelectric polarity opposite that of the carrier particles. As the mixture cascades or rolls across the image bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier particles and unused toner particles are then recycled. This technique is extremely good for the development of line copy images. The cascade development process is the most widely used commercial electrostatographic development technique. A general purpose office copying machine incorporating this technique is described in U.S. Pat. No. 3,099,943.
Another technique for developing electrostatic latent images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method a developer material containing toner and magnetic carrier particles is carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carriers in a brushlike configuration. This "magnetic brush" is engaged with an electrostatic latent image bearing surface and the toner particles are drawn from the brush to the electrostatic latent image by electrostatic attraction.
Another technique for developing electrostatic latent images is the "touchdown" process as disclosed, for example, in U.S. Pat. Nos. 2,895,847, and 3,245,823 to Mayo. In this method a developer material is carried to a latent image bearing surface by a support layer such as a web or sheet and is deposited thereon in conformity with said image.
It has been ascertained that in order to develop a latent image comprised of negative electrostatic charges, an electroscopic powder and carrier combination should be selected in which the powder is triboelectrically positive to the granular carrier; and to develop a latent image comprised of positive electrostatic charges, an electroscopic powder and carrier should be selected in which the powder is triboelectrically negative to the carrier. It is often desirable in any type of printing to produce a reverse copy of an original. By this is meant to produce a negative copy from a positive original or, on the other hand, a positive copy from a negative original. In electrostatographic printing, image reversal can be accomplished by applying to the image a developer powder which is repelled by the charged areas of the image and adheres to the discharged areas.
The triboelectric relationship between the electroscopic powder and the carrier depends on their relative positions in a triboelectric series in which the materials are arranged in such a way that each material is electrostatically charged with a positive charge when contacted with any material below it in the series and with a negative charge when contacted with any material above it in the series. In the reproduction of high contrast copy such as letters, tracings, etc., it is desirable to select the electroscopic powder and carrier materials so that their mutual electrification is sufficient to cause the toner particles to electrostatically cling to the carrier surface, and the degree of such electrification is normally governed by the distance between their positions in the triboelectric series, that is, the greater distance they are removed from one another, the greater the mutual electrification; and the closer they are together in the series, the less the mutual electrification.
Suitable coated and uncoated carrier materials for cascade, magnetic brush, and touchdown development are well known in the art. The carrier comprises any suitable solid material, provided that the carrier acquires a charge having an opposite polarity to that of the toner particles when brought in close contact with the toner particles so that the toner particles adhere to and surround the carrier. By proper selection of materials in accordance with their position in the triboelectric series, the polarities of their charge when the materials are mixed are such that the electroscopic toner particles adhere to and are coated on the surface of a carrier and also adhere to that portion of the electrostatic image bearing surface having a greater attraction for the toner than the carrier.
Carrier particles are generally made from or coated with materials having appropriate triboelectric properties as well as certain other physical characteristics. Thus, the materials employed as the carrier particles or the coatings thereon should have a triboelectric value commensurate with the triboelectric value of the toner to enable electrostatic adhesion of the toner to the carrier particles and subsequent transfer of the toner from the carrier particles to the image on the plate. Furthermore, the triboelectric properties of the carrier particles should be relatively uniform to permit uniform pickup and subsequent deposition of toner. The materials employed on the carrier particles should preferably have an intermediate hardness so as not to scratch the plate or drum surface upon which the electrostatic image is initially placed while being sufficiently hard to withstand the forces to which they are subjected during recycle. The carrier particles as well as the surface thereof also should not be comprised of materials which are so brittle as to cause either flaking of the surface or particle breakup under the forces exerted on the carrier during recycle. The flaking thereof causes undesirable effects in that the relatively small flaked particles will eventually be transferred to the copy surface thereby interfering with the deposited toner and causing imperfections in the copy image. Furthermore, flaking of the carrier surface will cause the resultant carrier to have nonuniform triboelectric properties when the carrier is composed of a material different from the surface coating thereon. This results in undesirable nonuniform pickup of toner by the carrier and nonuniform deposition of toner on the image. In addition, when the carrier particle size is reduced, the removal of the resultant small particles from the plate becomes increasingly difficult. Thus, the types of materials useful for making carrier or for coating carrier, although having the appropriate triboelectric properties, are limited because other physical properties which they possess may cause the undesirable results discussed above.
While ordinarily capable of producing good quality images, conventional developing materials suffer serious deficiencies in certain areas. The developing materials must flow freely to facilitate accurate metering and even distribution during the development and developer recycling phases of the electrostatographic process. Some developer materials, though possessing desirable properties such as proper triboelectric characteristics, are unsuitable because they tend to cake, bridge, and agglomerate during handling and storage. Adherence of carrier particles to reusable electrostatographic imaging surfaces causes the formation of undesirable scratches on the surfaces during the image transfer and surface cleaning steps. The tendency of carrier particles to adhere to imaging surfaces is aggravated when the carrier surfaces are rough and irregular. The triboelectric and flow characteristics of many carriers are adversely affected when relative humidity is high. For example, the triboelectric values of some carrier materials fluctuate with changes in relative humidity and are not desirable for employment in electrostatographic systems, particularly in automatic machines which require carriers having stable and predictable triboelectric values.
Another factor affecting the stability of carrier triboelectric properties is the susceptibility of carrier materials to "toner impaction". When carrier particles are employed in automatic machines and recycled through many cycles, the many collisions which occur between the carrier particles and other surfaces in the machine cause the toner particles carried on the surface of the carrier particles to be welded or otherwise forced into the carrier coatings. The gradual accumulation of permanently attached toner material on the surface of the carrier causes a change in the triboelectric value of the carrier and directly contributes to the degradation of copy quality by eventual destruction of the toner carrying capacity of the carrier.
Thus, there is a continuing need for a better developer material for developing electrostatic latent images.